The present invention relates to gas or combustion turbine apparatus, gas turbine electric power plants and control systems and operating methods therefor.
Industrial gas turbines may have varied cycle, structural and aerodynamic designs for a wide variety of uses. For example, gas turbines may employ the simple, regenerative, steam injection or combined cycle in driving an electric generator to produce electric power. Further, in these varied uses the gas turbine may have one or more shafts and many other rotor, casing, support, and combustion system structural features which can vary relatively widely among differently designed units. They may be aviation jet engines adapted for industrial service as described for example in an ASME paper entitled "The Pratt and Whitney Aircraft Jet Powered 121 MW Electrical Peaking Unit" presented at the New York Meeting in November-December 1964.
Other gas turbine uses include drive applications for pipeline or process industry compressors and surface transportation units. An additional application of gas turbines is that which involves recovery of turbine exhaust heat energy in other apparatus such as electric power or industrial boilers or other heat transfer apparatus. More generally, the gas turbine air flow path may form a part of an overall process system in which the gas turbine is used as an energy source in the flow path.
Gas turbine electric power plants are usable in base load, mid-range load and peak load power system applications. Combined cycle plants are normally usable for the base or mid-range applications while the power plant which employs a gas turbine only as a generator drive typically is highly useful for peak load generation because of its relatively low investment cost. Although the heat rate for gas turbines is relatively high in relation to steam turbines, the investment savings for peak load application typically offsets the higher fuel cost factor. Another economic advantage for gas turbines is that power generation capacity can be added in relatively small blocks such as 25 MW or 50 MW as needed for expected system growth thereby avoiding excessive capital expenditure and excessive system reserve requirements. Further background on peaking generation can be obtained in articles such as "Peaking Generation" a Special Report of Electric Light and Power dated November 1966.
Startup availability and low forced outage rates are particularly important for peak load power plant applications of gas turbines. Thus, reliable gas turbine startup and standby operations are particularly important for power system security and reliability.
In the operation of gas turbine apparatus and electric power plants, various kinds of controls have been employed. Relay-pneumatic type systems form a large part of the prior art. More recently, electronic controls of the analog type have been employed as perhaps represented by U.S. Pat. No. 3,520,133 entitled Gas Turbine Control System and issued on July 14, 1970 to A. Loft or by the control referred to in an article entitled Speedtronic Control, Protection and Sequential System and designated as GER-2461 in the General Electric Gas Turbine Reference Library. A wide variety of controls have been employed for aviation jet engines including electronic and computer controls as described for example in a March 1968 ASME Paper presented by J. E. Bayati and R. M. Frazzini and entitled "Digatec (Digital Gas Turbine Engine Control)", an April 1967 paper in the Journal of the Royal Aeronautical Society authored by E. S. Eccles and entitled "The Use of a Digital Computer for On-Line Control of a Jet Engine", or a July 1965 paper entitled "The Electronic Control of Gas Turbine Engines" by A. Sadler, S. Tweedy and P. J. Colburn in the July 1965 Journal of the Royal Aeronautical Society. However, the operational and control environment for jet engine operation differs considerably from that for industrial gas turbines. In referencing prior art publications or patents as background herein, no representation is made that the cited subject matter is the best prior art.
In connection with prior art gas turbine electric power plant operating and control systems and operating methods therefor, reference is made to copending related application Ser. No. 082,470 which in conjunction with the other enumerated related patent applications comprises a description of an improved gas turbine plant operating and control system. The present disclosure represents a further advancement over the prior art and the prior art discussion herein contained should be considered as exclusive of the referenced applications.
Generally, the operation of industrial gas turbine apparatus and gas turbine power plants have been limited in flexibility, response speed, accuracy and reliability. Limits have also existed on how close industrial gas turbines can operate to the turbine design limits over various speed and/or load ranges.
In gas turbine power plants, operational shortcomings have existed with respect to plant availability. Critical temperature limit control has been less protective and less responsive than otherwise desirable.
More particularly, in gas turbine controls exhaust temperature monitoring is essential since there exists an ever present danger of damage to combustor elements, hot parts, rotor blades, etc., in the event of over-temperature or overload conditions. Thermocouples have usually been placed in the exhaust gas stream to determine temperature of the gases discharged by the combustor elements through the turbine to thereby give an indication of an average discharge temperature. Such temperature monitoring increases turbine operation reliability and availability, serving to decrease maintenance inspections. The turbine thermocouples have provided a means by which turbine operators may manually initiate control actions including shutdown in the event of a safety hazard or indicated danger of serious damage to vital components.
As gas turbine automatic control systems developed, it became increasingly essential to obtain reliable temperature indications for use as control parameters in developing a fuel control input in the various control modes of operation. It became necessary to review the temperature measurements, not only for the purpose of assuring reliable, safe operation, but, further, to insure the availability of a control variable which would enable efficient operation of the gas turbine near design limits to thereby enhance overall efficiency of the automatic control system. More specifically, accurate, reliable exhaust temperature readings are essential to maintaining the integrity of a system having one or more control loops wherein it is sought to control turbine speed or load in response to a temperature derived fuel demand signal. During those modes of operation characterized principally by temperature control, the accuracy and reliability of such readings determines the degree to which optimum operating conditions may be obtained.
Various methods and apparatus exist for obtaining and displaying turbine exhaust temperature readings. Earlier temperature monitoring systems provided plural thermocouples connected in parallel yielding an average temperature indication for the combustion elements which could be displayed on a meter. A facility was provided for selecting a particular thermocouple for a reading if desired. However, the meter load influenced the average by taking the selected thermocouple value out. As may be readily appreciated, elimination of a thermocouple reading would have a decided effect in a system whose operation is at least partially determined as a function of such values, and would, therefore be unsuitable in a control loop.
More recently systems have been devised which employ plural thermocouples in a symmetrical arrangement for achieving high accuracy temperature sensing and averaging suitable for control loop implementation as well as monitoring and display. Representative of such a system is one wherein the temperature sensing is of exhaust gases near or within the exhaust gas cycle position or exhaust manifold. Control actions are determined in response to a control system input signal representing an average of the readings taken from a combination of the system thermocouples, usually all. This arrangement offers certain advantages over earlier prior art particularly insofar as selection of the combination of thermocouples for averaging is made while the control system is in operation. This is particularly desirable in the event that an indication is received that one or more of the thermocouples are open or shorted. As will be appreciated, the accuracy of the average obtained in such a system is influenced by the number of thermocouples selected. However, the location of the arrangement of thermocouples at the exhaust gas cycle position keeps the actual measure spread selectively small by virtue of the mixing of the gases prior to arrival at the exhaust location.
Although a reliable and accurate temperature reading average may be obtained for monitoring and control purposes in the foregoing manner, certain features essential to maintaining the integrity of an improved monitoring or alarm and control system to be used in conjunction with gas turbine control have not been provided heretofore. As hereinabove suggested limitations have existed in overall gas turbine operating flexibility, response speed, accuracy and reliability. Such limitations similarly have existed in gas temperature monitoring and control. To achieve such temperature limiting and control response as has been heretofore achieved, complicated arrangements of logic circuitry and redundant thermocouple input channels have been employed. Such protection and control system implementations are necessarily limited in the flexibility requisite to the monitoring and detection of system faults which are known to cause catastrophic failure of such vital systems components as the combustor baskets and rotor parts. Systems capable of determining all or substantially all conditions which may predictably contribute to such catastrophic failures have not as yet been devised. Nor is there suggested in the known prior art temperature monitoring and control systems a facility for indicating which turbine control or operating conditions or problems may be the underlying cause of determined thermocouple readings and the reliability and safety of gas turbine operation has been limited accordingly.
Gas turbine operating and control problems which may be associated with thermocouple process temperature readings include improperly functioning thermocouples. Such malfunctions are varied in cause and effect. An open thermocouple which has significant impact on attempts to obtain a reliable control average may result from a circuit break. Shorted thermocouples which may result from twisted wires characteristically create ambient junctions of low temperature, again, serving to decrease reliability. Grounded thermocouples may result in intermittent faulty readings thereby introducing uncertainty into computations. Reversed connections, a problem significant from the standpoint of frequency of occurrence as well as impact on the control system may cause the introduction of a large negative number into control average computations thereby occassioning the supply of fuel to the combustor elements considerably in excess of requirements.
Turbine system faults or failures which cause significant temperature related operating and control problems include a plugged combustor nozzle, poor interconnection of combustor baskets, and improper fuel atomization and combustor basket deterioration. As previously discussed resultant over-temperature conditions in the gas turbine elements may cause turbine damage such as damage to or failure of the combustor elements or rotor blades.